The invention relates to a process for determining an echo coupling factor and the echo delay time in a transmission system wherein an undesired coupling occurs between a receiving path and a transmitting path. This coupling gives rise to disturbing echos, the amplitude and delay time of which must be determined in order to be able to define the parameters of the transmission system and thus obtain a possible means of successfully compensating the echos. Echos occur as line echos in telecommunications systems due to electric coupling between the transmitting path and the receiving path and/or for example as acoustic echos in the case of handsfree telephones or auditorium sound systems due to acoustic coupling between loudspeaker and microphone. The degree of the coupling is defined by a coupling factor CF which indicates the ratio of the signal energy received at the echo reception location Y to the signal energy transmitted at the echo source X. Measured in dB, this ratio is also referred to as echo return loss ERL. For a sampled system the coupling factor CF is:                     CF        =                                            ∑                              k                =                0                            N                        ⁢                                          (                                  y                  ⁡                                      (                    k                    )                                                  )                            2                                                          ∑                              k                =                0                            N                        ⁢                                          (                                  x                  ⁡                                      (                                          k                      -                      i                                        )                                                  )                            2                                                          (        1        )            
wherein                               x          ⁡                      (                          k              -              i                        )                          =                  sampled          ⁢                                    xe2x80x83                        ⁢                          xe2x80x83                                ⁢          value          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          the          ⁢                                    xe2x80x83                        ⁢                          xe2x80x83                                ⁢          transmitted          ⁢                      xe2x80x83                    ⁢          signal                                                              y            ⁡                          (              k              )                                =                      sampled            ⁢                          xe2x80x83                        ⁢            value            ⁢                          xe2x80x83                        ⁢            of            ⁢                          xe2x80x83                        ⁢            the            ⁢                          xe2x80x83                        ⁢            received            ⁢                          xe2x80x83                        ⁢            signal                          ,                  echo          ⁢                      xe2x80x83                    ⁢          signal                                        N        =                  number          ⁢                                    xe2x80x83                        ⁢                          xe2x80x83                                ⁢          of          ⁢                      xe2x80x83                    ⁢          sampled          ⁢                                    xe2x80x83                        ⁢                          xe2x80x83                                ⁢          values          ⁢                                    xe2x80x83                        ⁢                          xe2x80x83                                ⁢          of          ⁢                                    xe2x80x83                        ⁢                          xe2x80x83                                ⁢          the          ⁢                      xe2x80x83                    ⁢          exciting          ⁢                      xe2x80x83                    ⁢          signal                                                  i          =                      echo            ⁢                          xe2x80x83                        ⁢            delay            ⁢                          xe2x80x83                        ⁢            time                          ,                  digitalized          .                    
The echo signal y(k) is delayed by i sample values compared to the transmitted signal x(kxe2x88x92i), namely by the delay time of the echo. An essential problem in determining the coupling factor CF consists in determining the echo delay time i in Equation (1) as accurately as possible. To determine the echo delay time i it is necessary to ascertain when the transmitted signal arrives at the echo reception location. As telecommunications systems are generally operated bidirectionally, the echo signal can be superimposed by a signal fed-in as useful signal at the remote end of the transmission path. This is the case whenever speech occurs simultaneously at the near end and the remote end of the transmission path. This is the so-called double talk situation. In the calculation of the coupling factor CF it must be ensured both that the echo delay time i is correctly set and that no double talk is occurring.
To detect an echo it is known to perform a correlation analysis in which the known transmitted signal x(k) is recorded in a time interval k=0 . . . N and compared with the signal y(k) incoming at the echo reception location, see DE-A-42 29 910. The degree of correlation xcfx81 then indicates the probability as to whether an echo xcfx81≈1 or a mixed signal xcfx81≈0 is present. The decision as to whether the correlation degree xcfx81 is to be interpreted as an echo or a mixed signal produced by double talk, is made by a threshold value xcex3. The greater the selected threshold value xcex3, the less often are existing echos detected. Thus incorrect assessments are often made and the adaptation of the system is blocked. If, on the other hand, the selected threshold value xcex3 is too small, the danger exists that the system will be adapted in double talk operation and become unstable. It is also disadvantageous that similarities between speech signals likewise give rise to incorrect decisions. Frequently spoken vowels can originate both from the echo generating subscriber and from the subscriber generating the double talk. The calculation outlay for the correlation analysis, comprising N multiplications, N additions and one division, is very high.
A process is also known for calculating a coupling factor in handsfree telephone systems in which, in order to reduce the calculation outlay, short-term mean values of the transmitted and received signals are obtained using recursive filters, see DE-A-44 30 189. Here the fact is exploited that the double talk times in a normal conversation are limited and the coupling factors in the case of double talk are always greater than in the case of a monologue. The specification of a minimum coupling factor in a time interval of several seconds eliminates incorrect decisions due to a temporary increase in the coupling factor in the case of double talk. The basic delay of the handsfree system is determined by the distance between sound generator and sound receiver and by the delay time due to the A/D, D/A converters, and is approximately constant. The coupling factor assessment error is reduced if the exciting signal is greater than a quiescent threshold value, at least in a defined time interval, whereby temporary disturbances are excluded from the assessment.
However, the precondition that the basic delay of the echo signal is approximately constant no longer applies in modern telecommunications systems and therefore the known processes do not produce the desired success.
The invention now achieves the object of providing a process for determining the echo coupling factor for telecommunications systems with echo delay times which change within a wide range, in order to achieve a required, rapid adaptation of an echo compensator. Particularly large echo delay times occur in the case of satellite transmission links and radio transmission links, for example according to the GSM standard, with a plurality of coder- and decoder circuits. To be able to implement cost-efficient echo compensator circuits, the calculation outlay for determining the echo coupling factor and echo delay time is to be as small as possible.
This object is achieved in accordance with the invention by a process wherein, by analyzing a dialogue, states and time segments in which the probability of an incorrect assessment is very low are determined. The echo delay time and echo coupling factor can be reliably determined only when the echo-generating subscriber starts speaking after a speech pause, either on his own part or on the part of the subscriber at the remote end of the transmission link. During a conversation it is not easily possible to make a reliable pronouncement on the properties of the transmission link.